by Dana Moses

Created on: May 12, 2009   Last Updated: May 13, 2009

Photosynthesis is the biochemical process that uses carbon dioxide, water, and energy from sunlight to produce oxygen and sugar. Photosynthesis occurs in plants and some algae and is the ultimate source of all the food we eat.

Plant leaves are the primary site of photosynthesis in green plants. Stomata are pores in the leaves that allow carbon dioxide into the leaf, and also allow oxygen to be released into the environment. Leaf cells contain specialized compartments, or organelles, called chloroplasts, which contain pigments. The most abundant and best known pigment in chloroplasts is chlorophyll, but chloroplasts also contain carotenoids and other pigments. Different pigments absorb light energy best at different wavelengths (or colors), enabling a single chloroplast to receive energy from a variety of wavelengths of light and also letting us enjoy brilliant leaf colors as chlorophyll fades from leaves in the fall.

The energy absorbed by the pigments is transferred to pairs of special chlorophyll molecules in what are called the reaction centers. There are 2 reaction centers in photosynthesis: the first is where oxygen is made from water, and the second is where a chemical called NADPH is made, which is essential for fixing carbon dioxide into sugars. The absorbed light energy causes an electron in the chlorophyll of the reaction center to be excited from its low-energy, ground state to a higher energy state. The excited electron is then passed to a series of electron acceptors: pheophytin, plastoquinone, the cytochrome complex, and plastocyanin all accept the electron in a chain. Plastocyanin then gives the electron to the chlorophyll pair in the second reaction center, which passes the electron to another series of electron acceptors, including a quinone, a set of iron-sulfur clusters, ferredoxin, and finally to NADP+ to form NADPH. The electron that the first special chlorophyll pair donates to the chain is replaced by an electron from water, producing oxygen.

Altogether, the 2 reaction centers use energy from light to consume water and NADP+ and to produce oxygen and NADPH. In addition, several of the electron acceptors in the electron transfer chain displace protons across a membrane in the chloroplasts, which is used to create ATP, the cell's chemical energy source.

The second phase of photosynthesis, known as the dark reactions or the Calvin cycle, uses the ATP and NADPH produced in the first part of photosynthesis to fix carbon dioxide into sugar. The first reaction of the Calvin cycle is carried out by an enzyme called RuBisCO, which is short for ribulose-1,5-bisphosphate carboxylase. RuBisCO is the most abundant enzyme on the planet. RuBisCO adds carbon dioxide to a 5-carbon sugar, ribulose-1,5-bisphosphate, to form a 6-carbon sugar which immediately splits into 2 molecules of 3-phosphoglycerate, a 3-carbon sugar. Subsequent reactions of the Calvin cycle use ATP and electrons from NADPH to convert 3-phosphoglycerate into glyceraldehyde-3-phosphate. Glyceraldehyde-3-phosphate can be reversibly converted to another 3-carbon molecule, dihydroxyacetone phosphate.

After 3 rounds of the above reactions, 6 molecules of 3-carbon sugars have been produced. The remainder of the Calvin cycle consists of a complex series of reactions that rearrange 5 of these 6 molecules into 3 molecules of ribulose-1,5-bisphosphate. The one remaining molecule of glyceraldehyde-3-phosphate is available to be combined with another 3-carbon molecule to make a 6-carbon sugar, or a hexose. Hexoses are the basis for forming larger sugar molecules, called polysaccharides, which include glycogen, starch, and cellulose.

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